At the manufacture of cellulose pulp from wood, for example at the manufacture of mechanical or chemi-mechanical pulp, a part or the main part of the resin acids and higher fatty acids of the wood, depending on the process conditions, are solved or dispersed.
These resin acids and fatty acids and the sodium salts thereof have proved at experiments with anaerobic treatment of the wastewater to be toxic toward methane bacteria. Tests, on which the present invention is based, have shown that it is possible by means of additives to inhibit these toxic effects.